fix(overlay): style inspector button differently when enabled

[skirtles-code]

As suggested in #1067, this PR changes the styling of the Toggle Component Inspector button, so that it only shows in green when it is enabled. This is similar to how the Toggle Vue DevTools button is styled.

Most of the code to do this already existed, but there was some ambiguity about what it means for the Vue inspector to be 'enabled'. I've split this into two separate things: vueInspectorSupported and vueInspectorEnabled. The former signifies whether the button can be shown, whereas the latter indicates that the button has been clicked and the user is actively trying to select a component.

[netlify]

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commit: 58a61c8